Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a microscopic level within the finished structure. Fiber reinforced composites are composed of fibers dispersed across a matrix system. The matrix can be polymer, ceramics, metal, or graphite. The fiber carries the load due to its high strength and high modulus and the matrix maintains the fiber(s) in proper position and transfers the load between the fibers. Fiber reinforced composites are well-known in the materials industry and are routinely used in aerospace, sporting goods, energy, medical, and automotive applications due to their superlative mechanical properties, low densities, and sometimes for their thermal and chemical and weather resistance.
Notwithstanding the brilliant theoretical properties of composite materials, the true material properties observed in the real world fall short of those theoretically predicted. Reasons for this can be found but not limited in: (1) the interface between the fiber and the matrix is weaker than the bulk properties of the matrix and the fiber; (2) the fiber sometimes buckles in the micro-scale and cannot take the loading as theoretically expected; (3) manufactured defects of the composites; (4) unexpected loadings (such as bird impact) on the composites that cause undesired and undetected internal damages (such as but not limited to the delamination, matrix crack, interface crack, etc.) on the composites; (5) reversible or irreversible property change of the matrix system due to temperature change or other environmental changes or aging problems; (6) residual stress/strain resulting from the processing and assembly of the composite structure itself or the use of the composite structure in a raised or lowered temperature. In short, there are many different mechanisms for causing the reduction of mechanical performance of composite structures. Importantly, as the major load-carrying system is the fiber, it follows that any change in the load behaviour of the fiber system will also cause a change (typically a reduction) in the mechanical performance of a composite during the service life of a composite structure.
Although there are many prior art ways to measure the total deformation of a composite structure, Applicant has determined that there are not any effective and efficient methods that can measure the deformation (or strain) of a fiber inside a composite during the use of the composite structure. The technical challenge is caused by (1) the typically small micro-size diameter of the fiber, (2) the distinctive thermal and mechanical properties between the fiber and matrix, and (3) the presently unknown and unmeasurable interface behavior between the fiber and the matrix.
One of the major causes of different strains between the fiber and the matrix is residual stress or residual strain. Residual stress is inherently present in almost all composite materials. The residual strain tends to affect both the physical and mechanical characteristics of the composite materials and is responsible for dimensional instability and an increase in damage to and premature failure of the composite. Accordingly, it is imperative that thermal residual stresses be taken into account when designing composite structures, in particular carbon fiber polymer matrix composites.
Analytical modeling of composite structures has evolved tremendously over the past few years. These techniques are applicable to continuous fiber reinforced thermoplastic composites and they provide an estimation of the magnitude of the thermal residual stresses on three (3) levels. On the micro-mechanical level or constituent level the operative parameter turns out to be the mismatch in the coefficient of thermal expansions between the reinforcing fibers and the polymer matrix. This results in compressive residual strains in the fiber and tensile strains in the surrounding polymer matrix, and can also cause macro-mechanical or lamination residual stresses. Such macro-mechanical residual stresses are present on a ply-to-ply scale due to lamina anisotropy (a difference in the transverse and longitudinal ply coefficients of thermal expansion). On a global laminate level, a gradient in cooling rate and temperature throughout the thickness of the composite laminate or structure may result in a residual stress distribution through the thickness of the laminate. These levels may also be referred to as the intralaminar, interlaminar and laminate stresses.
Prior art techniques attempt to evaluate the residual stress of a composite laminate. These techniques can be divided into three categories: first methods using intrinsic composite constituents material properties, second methods employing embedded ‘foreign” stress sensors and third techniques based on in-plane and out-of-plane deformations.
Techniques using intrinsic material properties include photo-elasticity which is a well-known optical technique for static stress analysis. For determination of the stress field in composites, however, a transparent or translucent matrix is necessary. This method has been applied to the transparent thermoset matrix composites and amorphous thermoplastic matrices. In amorphous polymers, stress alters the molecular orientation distribution, thereby affecting the polarization state of the light. When seeking to determine the magnitude of the residual stress, the measurement of retardation, i.e., phase difference between two light vectors moving at different velocities, is calculated using Brewster's Law. A disadvantage of this method is the requirement of thin composite layers with low fiber volume.
Another technique for residual strain determination utilizing intrinsic material property is micro-laser Raman spectroscopy. This technique is based on the stress sensitivity of most Raman vibrational modes of crystalline phases. The difference in energy between the incident photon and the Raman scattered proton is equal to the energy of the vibration of the scattering molecule. A plot of intensity of scattered light versus energy difference is a Raman spectrum. Raman spectroscopy is a well known method for measuring the state of strains in carbon fibers embedded in a translucent polymer matrix, since certain peaks positions in the Raman spectrum of the fiber change with applied strain. The “Raman peaks” of the fiber. There is a problem however. The Raman spectra are taken from the laminate surface and fiber surfaces and differences in fiber and laminate skin and core behavior upon straining can and do exist. In addition, certain amorphous fibers have a very week Raman response and can not be readily used as intrinsic stress or strain sensors, for example glass.
In the carbon fiber/polymer matrix composites electrical properties, for example resistance, can also be affected by strain and temperature. Therefore, the electrical resistance of a carbon fiber/polymer matrix can be monitored to indicate strain damage without the requirement for “foreign” sensors. As a result, inter-laminar residual stresses can be determined by measuring electrical resistance of cross ply laminates. It is well known that upon curing of thermoset matrix composites at increased pressure, the electrical conductivity barrier, measured as the activation energy, increased. Thermosetting plastics (thermosets) are polymer materials that irreversibly cure. The cure may be done through heat (generally above 120 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.
The resin curing can attribute to increased residual interlaminar stress. Due to these stresses, the quality of the interlaminar interface in angle-ply laminates breaks down. If the bond achieved at high temperatures is not sufficient to withstand de-bonding due to thermal stresses formed during subsequent cooling then the consequence is a higher electrical resistance measurement during cooling. The disadvantage of this method is that in order to accurately quantify the residual stresses present more information is required.
Another method to determine and follow thermal strain build up during cooling is to embed strain sensors in the composite itself. Typical sensors used for this purpose include, but are not limited to, strain gauges, fiber optic sensors and embedded metallic particles in combination with X-ray diffraction. A measurable change in properties have been shown by these sensors when exposed to residual thermal strains, to the extent adequate mechanical interaction exists between the composite and the sensor. One example of this technique involves the use of strain gauges to direct measure. Residual strain developments attributable to crystallization effects found in unidirectional laminates (CF/PEEK). In thermset composites, embedded strain gauges have been shown to provide accurate results during both cooling and heating.
It is not uncommon for fiber optic sensors to be utilized as internal “strain gauges” to follow the development of thermal residual strains within a composite laminate, particularly when processing during the high temperatures required for thermoplastic composites. This technique can also be applied to unidirectional laminates as well in angle ply laminates.
A number of different fiber optic sensors are available for use in this technique, for example the fiber Bragg grating (FBG) and the extrinsic Fabry-Perot interfermettic (EFPI) sensors. An EFPI sensor measures strain through a change in cavity length, which is related to a phase change between the input/output signals and the reflection of the optical fibers. The disadvantage of this method is that a cavity (and, as a consequence, a weakness) is included in the laminate and the diameter of the sensor is so large that stress concentrations may arise. Furthermore, the EFPI sensor is prone to failure caused by thermal residual stresses alone.
FBG sensors show much greater promise for monitoring residual strain because their diameters are small and accurate. The FBG sensor operates on the response to strain and temperature differences by a change in Bragg wavelength. The reflection spectra of the optical sensors may split into two peaks due the non-axisymmetric residual stresses in the composite. As a result, polarization effects and strain differences throughout the length of specimen can be detected. Wave length changes can be used to determine thermal residual strains with relations based on the photoelastic constant and effective refraction index of the optic fiber. The disadvantage of utilizing optic fiber sensors is that when it is embedded perpendicular to the fiber, an eye-shaped defect results thereby causing a significant stress concentration and decrease in mechanical properties.
Another of the “embedded sensor” techniques includes using X-ray techniques on metallic particle inclusions in the matrix and thereby measuring the diffraction imposed by residual strains in the polymer matrix. Embedded metal particles, such as copper, aluminum or silver, show a deflection in peak angle when embedded in a composite. Utilizing Braggs law, this deflection is related to a change in crystal lattice spacing caused by the residual strain. The measured strain can be related to the residual stress in the polymer matrix using Hooks law. Intralaminar and interlaminar residual strain detection in unidirectional laminate has been found to be possible. Typically this type of technique is used with thermoset matrix materials due to the fact that thermosets do not have crystallized structure that changes in response to X-rays when strained and therefore require crystalline fillers. However, in semi-crystalline thermoplastic, the lattice spacing between the crystals and the change due to straining can be followed by means of X-ray diffraction. The key disadvantage of this technique is that it either gives information on the surface properties of the sample only or, at best, only a thin portion of the sample.
Some prior art techniques for measuring residual stress in composites are based on in-plane and out-plane and out-of plane deformations. One of these methods is known as an interferometry-based method. This method uses the phenomenon of interference of light waves reflecting from a sample. This interference causes a visual fringe pattern that can be used to determine deformations. Some interference-based methods have been used to determine residual stress formation in composites. A well-known effect in optics is the Moire effect. It is based on an interference pattern that develops when light passes through two gratings that are rotated over a small angle with respect to one another. When one of the gratings changes due to deformation of the sample, the resulting interference pattern will change as well. This type of interference can be used for measuring displacements, both in-plane and out-of-plane. For measuring in-plane displacement, a grating should be applied to the surface of the sample. The grating can be projected onto the surface at an angle to the viewing direction.
The cure reference method was developed to determine the thermal strain development in thermoset composites with Moire interferometry. This method is a full-field laser based optical technique of Moire Interferometry to monitor strains on the surface of the thermoset laminate that initiate during cooling. The grating, which is applied during consolidation, acts as a reference to the stress free conditions prior to the stress free temperature. A characteristic pattern of light and dark fringes results. This pattern can be used to determine the in-plane displacements in symmetric laminates from which the residual can be calculated. Although accurate, this method only gives information of the residual strain state on the surface. In addition, an interference image needs to be captured when no strains are present.
A common manifestation of residual stress is warping of laminates with unsymmetrical lay-up. Accordingly, the use of unsymmetrical cross-ply or angle ply laminate can be used to determine the magnitude of the residual stresses of plies, the reason being that these residual stresses can partially be relieved by out-of-plane deformation. The out-of-plane deformations of a cross-ply or angle-ply laminate can be monitored during or after cooling from the processing temperature. The higher the curvature for a certain laminate with a particular thickness, the higher the residual stresses. At a certain temperature, the residual stress that would exist perpendicular to the fiber in a corresponding symmetrical cross-ply laminate can be calculated from the curvature, based on linear lamination theory.
Another approach is to compare the obtained curvatures with curvature predictions based on classical lamination theory. When the theory can accurately predict the curvatures, it may be assumed that the calculated residual stresses are also accurate and also that appropriate thermelastic properties of each lamina were used, including composite stiffness characteristics, thermal expansion coefficients and the difference between the service temperature and the temperature at which residual stresses start to build up. One drawback of the curvature method is that values of curvature may show variations for equal laminates under similar conditions.
Patents relevant to measuring thermal residual stress include: Viertl, et al, U.S. Pat. No. 4,249,423, which discloses residual stress measurements made using a strain gage and stress relief; Paton, et al., U.S. Pat. No. 6,072,568, which relates to a non-destructive measurement method for determining residual stress proximate an intermediate layer in a multilayer thermal barrier coating system; LeVert, et al., U.S. Pat. No. 6,353,656, disclosing a radioisotope based x-ray residual stress analysis apparatus having a shielded, monoenergetic radioisotopic source to emit x rays for measurement of the stress state of a polycrystalline material; Sampath, et al. U.S. Pat. No. 6,478,875; relates to an apparatus for performing in-situ curvature measurement of a substrate during a deposition process which includes a clamp for retaining the substrate near one end while leaving the opposite end free; and Prevey, III, U.S. Pat. No. 5,826,453 which relates to utilizing a single-point burnishing process to provide deep compression with a minimal amount of cold working and surface hardening. In particular, the area to be burnished along the surface of a workpiece is defined and a freely rotating burnishing ball is forced against the surface of the workpiece to produce a zone of deformation having a deep layer of compression within the surface.
Other patents of which the Applicant is aware include: U.S. Pat. No. 3,779,071 to Thomas Jr., et al. discloses a conductive wire based fatigue strain gauge including glass fibers in a resin matrix glass fiber composite; U.S. Pat. No. 5,379,644 to Yanagida discloses a bundled carbon fiber strain gauge consisting of many carbon fibers that are simply included in the matrix and is thus unable to distinguish or isolate themselves from surrounding fibers; and U.S. Pat. No. 6,277,771 to Nishimura discloses a carbon fiber and metal wire form sheet where the wire is not more than four percent and the wire is insulated.
Hence, it is clear that the prior art as extensive as it is still lacks: (1) an in-situ sensor method to experience the strain that can effectively represent the strain of the micro-diameter fiber during the use of the composite structure at different temperatures (2) an in-situ sensor to measure the residual stress of a composite caused by the manufacturing process and (3) an in-situ sensor to monitor the composite performance and health during the service life of a composite structure.
It, therefore, is an object of this invention to provide an insulated fiber sensor apparatus and method that is part of a composite material, including both the fiber system and the matrix system, but separate from it and yet which is sufficiently close to the dimension of the fibers in the matrix that it does not create voids or weaknesses in the matrix. It is a further object of the invention to provide an insulated fiber sensor that when connected with a measurement device provides the user with real time in-situ data concerning strain, temperature and compression.